Nickel Based Superalloys and Articles

ABSTRACT

Nickel based alloys are provided comprising from about 7.0 weight percent (wt %) to about 12.0 wt % chromium, from about 0.1 wt % to about 5 wt % molybdenum, from about 0.2 wt % to about 4.5 wt % titanium, from about 4 wt % to about 6 wt % aluminum, from about 3 wt % to about 4.9 wt % cobalt, from about 6.0 wt % to about 9.0 wt % tungsten, from about 4.0 wt % to about 6.5 wt % tantalum, from about 0.05 wt % to about 0.6 wt % hafnium, up to about 1.0 wt % niobium, up to about 0.02 wt % boron, and up to about 0.1 wt % carbon, with the remainder being nickel and incidental impurities. The alloys may be cast, directionally solidified and heat treated to provide articles having a gamma prime fraction of greater than about 50%.

This application is a divisional application of copending U.S. patentapplication Ser. No. 12/551,294 and claims priority to U.S. patentapplication Ser. No. 12/551,294, the content of which is incorporated byreference herein by its entirety.

BACKGROUND

The present disclosure relates to nickel-based alloys and articles basedthereupon.

Gas turbine engines operate in extreme environments, exposing enginecomponents, especially those in the turbine section, to high operatingtemperatures and stresses. Power turbine buckets (or blades) inparticular, which may be up to, or over, about 36 inches long and weightup to, or over, about 40 pounds, require a balance of propertiesincluding, but not limited to, casting cracking resistance, tensilestrength, ductility, creep resistance, oxidation resistance, hotcorrosion resistance, low freckle susceptibility, sufficiently lowdensity, reasonable cost, and a moderately large heat treatment window.

Superalloys have been used in these demanding applications because oftheir ability to maintain reasonably high strengths up to ˜75% of theirrespective melting temperatures, in addition to having excellentenvironmental resistance. Nickel-based superalloys, in particular, havebeen used extensively throughout gas turbine engines, e.g., in turbineblade, nozzle, and shroud applications. However, conventionalnickelbased superalloys used in latter stage bucket applications can bedifficult to cast, resulting in low yield. The steady increase in gasturbine firing temperature requirements has historically relied uponimproved mechanical and environmental material performance in theseapplications.

Directional solidification has been successfully employed to optimizecreep and rupture behavior in nickel-based superalloy bucketapplications. Preferentially orienting grains in the direction of theprincipal stress axis, which generally coincides with the longitudinaldirection, provides a columnar grain structure, eliminating grainboundaries transverse to the growth direction. Such an orientation alsoprovides a favorable modulus of elasticity in the longitudinaldirection, beneficial to the fatigue performance of the part.

When compared with conventionally cast alloy articles, the applicationof the directional solidification process produces articles havingsignificant improvements in strength, ductility, and resistance tothermal fatigue. However, reduced strength and ductility properties maystill be seen in the transverse direction in such articles due to thepresence of columnar grain boundaries. In efforts to improve thetransverse grain boundary strength of such articles, additional alloyingelements, e.g., hathium, carbon, boron and zirconium, have beenutilized. However, the addition of these elements, as well as others,can result in a depression in other desired properties, e.g., meltingpoint, and so a compromise in the balance of properties has heretoforebeen required.

Thus, there remains a need for nickel based alloys that exhibit more, orsubstantially all, of the desirable properties for use in gas turbineengines, e.g., resistance to corrosion, oxidation and creep, as well ashigh temperature strength. It would further be desired if any alloys soprovided would either comprise elements not substantially detrimental tothe desired properties, or be processed in such a way that any detrimentto the desired properties is minimized, or eliminated.

BRIEF DESCRIPTION

There are provided herein nickel-based alloys comprising from about 7.0weight percent (wt %) to about 12.0 wt % chromium, from about 0.1 wt %to about 5 wt % molybdenum, from about 0.2 wt % to about 4.5 wt %titanium, from about 4 wt % to about 6 wt % aluminum, from about 3 wt %to about 4.9 wt % cobalt, from about 6.0 wt % to about 9.0 wt %tungsten, from about 4.0 wt % to about 6.5 wt % tantalum, from about0.05 wt % to about 0.6 wt % hafnium, up to about 1.0 wt % niobium, up toabout 0.02 wt % boron, and up to about 0.1 wt % carbon, with theremainder being nickel and incidental impurities.

There are also provided herein nickel-based alloys comprising from about9.0 wt % to about 11.0 wt % chromium, from about 0.5 wt % to about 3.0wt % molybdenum, from about 0.5 wt % to about 3.5 wt % titanium, fromabout 4 wt % to about 6 wt % aluminum, from about 3.5 wt % to about 4.25wt % cobalt, from about 6.0 wt % to about 9.0 wt % tungsten, from about4.0 wt % to about 6.5 wt % tantalum, from about 0.05 wt % to about 0.5wt % hafnium, up to about 1.0 wt % niobium, up to about 0.01 wt % boron,and up to about 0.07 wt % carbon, the balance being nickel andincidental impurities.

A cast article is also provided and in one embodiment is formed from anickel based alloy comprising from about 7.0 weight percent (wt %) toabout 12.0 wt % chromium, from about 0.1 wt % to about 5 wt %molybdenum, from about 0.2 wt % to about 4.5 wt % titanium, from about 4wt % to about 6 wt % aluminum, from about 3 wt % to about 4.9 wt %cobalt, from about 6.0 wt % to about 9.0 wt % tungsten, from about 4.0wt % to about 6.5 wt % tantalum, from about 0.05 wt % to about 0.6 wt %hafnium, up to about 1.0 wt % niobium, up to about 0.02 wt % boron, andup to about 0.1 wt % carbon, with the remainder being nickel andincidental impurities. The cast article has a gamma prime fraction ofgreater than about 50%.

There is also provided a cast article formed from a nickel based alloycomprising from about 9.0 wt % to about 11.0 wt % chromium, from about0.5 wt % to about 3.0 wt % molybdenum, from about 0.5 wt % to about 3.5wt % titanium, from about 4 wt % to about 6 wt % aluminum, from about3.5 wt % to about 4.25 wt % cobalt, from about 6.0 wt % to about 9.0 wt% tungsten, from about 4.0 wt % to about 6.5 wt % tantalum, from about0.05 wt % to about 0.5 wt % hafnium, up to about 1.0 wt % niobium, up toabout 0.01 wt % boron, and up to about 0.07 wt % carbon, the balancebeing nickel and incidental impurities. The cast article has a gammaprime fraction of greater than about 50%.

In an additional embodiment there is provided a method for providing acast and heat treated article. The method comprises providing anickel-based alloy comprising from about 7.0 weight percent (wt %) toabout 12.0 wt % chromium, from about 0.1 wt % to about 5 wt %molybdenum, from about 0.2 wt % to about 4.5 wt % titanium, from about 4wt % to about 6 wt % aluminum, from about 3 wt % to about 4.9 wt %cobalt, from about 6.0 wt % to about 9.0 wt % tungsten, from about 4.0wt % to about 6.5 wt % tantalum, from about 0.05 wt % to about 0.6 wt %hafnium, up to about 1.0 wt % niobium, up to about 0.02 wt % boron, andup to about 0.1 wt % carbon, with the remainder being nickel andincidental impurities. The alloy is melted and directionally solidifiedto produce the article, and the article is heat treated so that thearticle comprises a gamma prime fraction of greater than about 50%.

In an additional embodiment there is provided a method for providing acast and heat treated article. The method comprises providing anickel-based alloy comprising from about 9.0 wt % to about 11.0 wt %chromium, from about 0.5 wt % to about 3.0 wt % molybdenum, from about0.5 wt % to about 3.5 wt % titanium, from about 4 wt % to about 6 wt %aluminum, from about 3.5 wt % to about 4.25 wt % cobalt, from about 6.0wt % to about 9.0 wt % tungsten, from about 4.0 wt % to about 6.5 wt %tantalum, from about 0.05 wt % to about 0.5 wt % hafnium, up to about1.0 wt % niobium, up to about 0.01 wt % boron, and up to about 0.07 wt %carbon, the balance being nickel and incidental impurities. The alloy ismelted and directionally solidified to produce the article, and thearticle heat treated so that the article comprises a gamma primefraction of greater than about 50%.

DETAILED DESCRIPTION

Unless defined otherwise, technical and scientific terms used hereinhave the same meaning as is commonly understood by one of skill in theart to which this invention belongs. The terms “first”, “second”, andthe like, as used herein do not denote any order, quantity, orimportance, but rather are used to distinguish one element from another.Also, the terms “a” and “an” do not denote a limitation of quantity, butrather denote the presence of at least one of the referenced item, andthe terms “front”, “back”, “bottom”, and/or “top”, unless otherwisenoted, are merely used for convenience of description, and are notlimited to any one position or spatial orientation. If ranges aredisclosed, the endpoints of all ranges directed to the same component orproperty are inclusive and independently combinable (e.g., ranges of “upto about 25 wt. %, or, more specifically, about 5 wt. % to about 20 wt.%,” is inclusive of the endpoints and all intermediate values of theranges of “about 5 wt. % to about 25 wt. %,” etc.). The modifier “about”used in connection with a quantity is inclusive of the stated value andhas the meaning dictated by the context (e.g., includes the degree oferror associated with measurement of the particular quantity).

A nickel-based superalloy is provided herein comprising a uniquecombination of alloying elements that result in the alloy beingparticularly adapted for casting and directional solidification toprovide articles, e.g., gas turbine buckets, having a combination ofimproved mechanical properties, as well as improved resistance tooxidation and hot corrosion. More particularly, articles formed from thesuperalloys described can exhibit improved casting cracking resistanceand a greater heat treatment window as compared to conventionalnickel-based superalloys, so that the cost of manufacture can be reducedand the yield of cast parts can be increased. Further, articles producedusing the present superalloys may also exhibit increased strength,ductility and creep resistance as compared to conventional Ni-basedsuperalloys, so that the articles may be used at higher operatingtemperatures, and/or have longer useful lives and/or, in the instance ofturbine buckets, be provided at longer lengths to provide improvedefficiency.

It is well known that alloying elements will typically partition betweenthe phases of an alloy in a manner related to the bulk chemistry. Aphase of an alloy is considered to be a homogeneous, physically andchemically distinct constituent that is separated from the remainder ofthe alloy by distinct bonding surfaces. The structure of the alloys,typical of nickel-based superalloys, comprises a major phase known asgamma, which is the matrix of the alloy and thus commonly referred to asthe gamma matrix. Alloy structure also comprises a major precipitatephase within the gamma matrix, called the gamma prime precipitate phase,and minor amounts of carbides, oxides, and borides. The high temperaturestrength of a nickel based superalloy is thought to be related to theamount of gamma prime precipitate phase present, in addition to thesolid solution strengthening of the gamma matrix.

The alloying elements partition between the phases with the mostimportant being the partitioning between the gamma matrix and the gammaprime precipitate. An understanding of how the elements partitionbetween phases is necessary in alloy design to permit calculation ofseveral alloy characteristics of importance including the chemicalcomposition of gamma, gamma prime, carbides, oxides, and borides; theamount of gamma prime present as gamma prime particles and asgamma-gamma prime eutectic; stability of the gamma phase; and atomiclattice mismatch between gamma and gamma prime.

An analysis of a number of superalloys has shown that among thosealloying elements generally used in the development of nickel-basedsuperalloys, elements partitioning to the gamma matrix and which act asgamma solid solution strengthening elements are chromium (Cr), cobalt(Co), molybdenum (Mo), tungsten (W), rhenium (Re), and iron (Fe). Ingeneral, the heavy (large atom) refractory elements such as rhenium,tungsten and molybdenum are the most effective strengtheners at hightemperatures. Solid solution strengthening is desirably achieved withoutcausing instability of the matrix structure. Instability, which can haveadverse effects on alloy properties, results from the development ofunwanted phases or precipitates at high temperatures. And so, suchphases or precipitates are desirably avoided.

The second major strengthening mechanism recognized in nickel-basedsuperalloys is precipitation hardening. The precipitate is formed withinthe gamma matrix and is known as gamma prime. Gamma prime is an orderedface-centered cubic compound, Ni₃Al, which is coherent with the nickelmatrix. Elements that segregate preferentially to the gamma prime phaseinclude aluminum (Al), titanium (Ti), tantalum (Ta), niobium (Nb), andvanadium (V).

The present nickel-based superalloys, in some embodiments, exhibitsuperior castability, high temperature strength and creep behavior,cyclic oxidation resistance, and hot corrosion resistance as compared toconventional nickel-based superalloys. The superalloys described arealso adapted for casting, directional solidification and heat treatmentto provide articles, e.g., gas turbine buckets, while retaining thebasic properties of the superalloy.

The nickel-based alloy designed accordingly and disclosed hereincomprises chromium, molybdenum, titanium, aluminum, cobalt, tungsten,tantalum, hafnium, niobium, boron, and carbon. The nickel-based alloy isdevoid of rhenium, thereby providing cost savings. In one embodiment,the nickel-based superalloy comprises from about 7.0 weight percent (wt%) to about 12.0 wt % chromium, from about 0.1 wt % to about 5 wt %molybdenum, from about 0.2 wt % to about 4.5 wt % titanium, from about 4wt % to about 6 wt % aluminum, from about 3 wt % to about 4.9 wt %cobalt, from about 6.0 wt % to about 9.0 wt % tungsten, from about 4.0wt % to about 6.5 wt % tantalum, from about 0.05 wt % to about 0.6 wt %hafnium, up to about 1.0 wt % niobium, up to about 0.02 wt % boron, andup to about 0.1 wt % carbon, with the remainder being nickel andincidental impurities.

In another embodiment, the nickel-based alloy comprises from about 8.5wt % to about 11.0 wt % chromium, from about 0.5 wt % to about 3.0 wt %molybdenum, from about 0.5 wt % to about 3.5 wt % titanium, from about 4wt % to about 6 wt % aluminum, from about 3.5 wt % to about 4.25 wt %cobalt, from about 6.0 wt % to about 9.0 wt % tungsten, from about 4.0wt % to about 6.5 wt % tantalum, from about 0.05 wt % to about 0.5 wt %hafnium, up to about 1.0 wt % niobium, up to about 0.01 wt % boron, andup to about 0.07 wt % carbon, the balance being nickel and incidentalimpurities.

In some embodiments, the chromium content of the nickel-based alloy maydesirably be between about 7 wt % to about 12 wt %, or from about 8.5 wt% to about 11 wt %. In some embodiments, a balance is desirablymaintained between chromium and aluminum so that the alloy can exhibitboth good oxidation and hot corrosion resistance. Data generated in theevaluation of certain alloys described herein showed that a narrow Cr:Alratio of from about 1.5 to about 2.5 provided the balance of propertiesrequired. And so, an appropriate range for aluminum in certain alloysdescribed can be from about 4 wt % to about 6 wt %.

The titanium content of certain of the alloys described herein maydesirably be between about 0.2 wt % to about 4.5 wt %, or from about 0.5wt % to about 3.5 wt %. Titanium is desirably present in theaforementioned amounts so that the Al:Ti ratio can be greater than about1, or 2, or 3, or even greater than about 4.

Tungsten is a viable alloying element for high temperature strength, andcan partition to either the gamma phase, or the gamma prime phase.Tungsten may be included in certain of the described alloys in amountsof from about 6.0 wt % to about 9.0 wt %.

Molybdenum can act like tungsten in certain of the inventive alloys, buthas a lower density. Molybdenum can be detrimental to environmentalresistance, although this can be minimized by balancing amounts ofchromium. In some embodiments, where chromium is present at from about 7wt % to about 12 wt %, or from about 8.5 wt % to about 11 wt %,molybdenum may desirably be included in amounts of from about 0.1 wt %to about 5 wt %, or from about 0.5 wt % to about 3.0 wt %, so that theadded strength benefit is seen without a substantial detriment toenvironmental resistance.

Tantalum partitions like titanium in nickel-based alloys, partitioningalmost entirely to the gamma prime phase. Tantalum can be preferred overtitanium in some embodiments, since tantalum has a higher melting pointthan titanium, and thus may not depress the melting point of the alloyas much as a similar amount of titanium. However, tantalum is a heavyelement having a much higher density than titanium and so by utilizingmore titanium than tantalum, a lighter article can be provided. Bearingthese considerations in mind, useful amounts of tantalum in certainembodiments of the described superalloys can be from about 4.0 wt % toabout 6.5 wt %, based upon the total weight of the alloy.

Cobalt can raise the solid solubility temperature of gamma prime,thereby increasing the temperature capabilities of alloys in which it isincluded. Cobalt can also contribute to structural stability of thealloy by inhibiting sigma phase precipitation. For these reasons, amongothers, in certain embodiments, the alloys described herein may includefrom about 3.0 wt % to about 4.9 wt %, or from about 3.4 wt % to about4.25 wt %, cobalt, based upon the total weight of the alloy.

Hafnium can be useful as a grain boundary strengthener and can provideincreased resistance to oxidation. And so, in some embodiments, thealloys described herein include hafnium in amounts of up to about 1.0 wt%, or from about 0.05 wt % to about 0.5 wt % hafnium. In certainembodiments, the alloys further comprise niobium, in amounts of up toabout 1 wt %.

The nickel-based alloy may be processed according to any existingmethod(s) to form components for a gas turbine engine, including, butnot limited to, powder metallurgy processes (e.g., sintering, hotpressing, hot isostatic processing, hot vacuum compaction, and thelike), ingot casting, followed by directional solidification, investmentcasting, ingot casting followed by thermo-mechanical treatment,near-net-shape casting, chemical vapor deposition, physical vapordeposition, combinations of these and the like.

In one manner of manufacturing a gas turbine airfoil from a nickel-basedalloy as described, the desired components are provided in the form ofpowder particulates, either separately or as a mixture and heated to atemperature sufficient to melt the metal components, generally fromabout 1350° C. to about 1750° C. The molten metal is then poured into amold in a casting process to produce the desired shape.

As mentioned above, any casting method may be utilized, e.g., ingotcasting, investment casting, high gradient casting or near net shapecasting. In embodiments wherein more complex parts are desirablyproduced, the molten metal may desirably be cast by an investmentcasting process which may generally be more suitable for the productionof parts that cannot be produced by normal manufacturing techniques,such as turbine buckets, that have complex shapes, or turbine componentsthat have to withstand high temperatures. In another embodiment, themolten metal may be cast into turbine components by an ingot castingprocess. The casting may be done using gravity, pressure, inert gas orvacuum conditions. In some embodiments, casting is done in a vacuum.

After casting, the melt in the mold may advantageously be directionallysolidified. Directional solidification generally results in elongatedgrains in the direction of solidification, and thus, higher creepstrength for the airfoil than an equiaxed casting, and is suitable foruse in some embodiments. In particular, the present alloys may be formedinto multi-grained directionally solidified components, designed toaccommodate many grains across the cross-section of the part, at a muchgreater yield than conventional single crystal nickel-based superalloys.That is, although small components may typically be made as a singlecrystal, many of the larger components of gas turbine engines may bedifficult to form as a true single crystal. And so, yield of thesecomponents in SC form may not be commercially useful. In contrast, theyield of a similarly sized multi-grained directionally solidified gasturbine component utilizing embodiments described herein, can be atleast about 80%, or from about 80% to about 100%.

Following directional solidification, the castings are cooled, e.g., asby any conventional cooling method. The castings comprising thenickel-based alloy may then be optionally subjected to different heattreatments in order to optimize the strength as well as to increasecreep resistance. Desirably, heat treatment will result in the castinghaving a gamma prime fraction of greater than about 50%, or even greaterthan about 60%. The heat treatment may generally include heating thecasting in vacuum to a temperature of about 2260° F. to about 2400° F.for 2 to 4 hours. The casting may then be cooled by a furnace cool invacuum, argon or helium at a cooling rate of about 15° F./minute toabout 45° F./minute to 2050° F., followed by gas fan cooling in vacuum,argon or helium at about 100° F./minute to about 150° F./minute to 1200°F. or below. Once below 1200° F., the articles can be cooled to roomtemperature at any cooling rate.

In some embodiments, the castings may be subjected to an agingtreatment. For example, the castings may undergo aging by heating undervacuum to 1975° F. for a period of 4 hours, furnace cooling to below1200° F., heating to from about 1600° F. to about 1650° F. for 4 to 16hours, followed by a furnace cool to room temperature.

The nickel-based alloys described herein may thus be processed into avariety of airfoils for large gas turbine engines. As mentioned above,the Ni-based alloys described here can exhibit improved casting crackingresistance and a larger heat treatment window than conventionalnickel-based superalloys, e.g., Rene' N4, thereby reducing the cost ofmanufacture and increasing the yield of cast parts. Articles formed fromthe disclosed alloys may further exhibit increased strength, ductility,and creep resistance, as well as oxidation and hot corrosion resistance.As a result, such articles may be used at higher operating temperaturesand/or exhibit longer useful lives than articles formed fromconventional nickel-based alloys.

Examples of components or articles suitably formed from the alloysdescribed herein include, but are not limited to buckets (or blades),non-rotating nozzles (or vanes), shrouds, combustors, and the like.Components/articles thought to find particular benefit in being formedfrom the alloys described herein include nozzles and buckets. Thesuperalloy can be used with various thermal barrier coatings.

One exemplary method of making a cast and heat treated article such as alarge power turbine bucket of a nickel-base superalloy of the presentdisclosure may generally proceed as follows. The desired component,e.g., a turbine bucket, may be directionally cast with the superalloy.The casting may then be subjected to a heat treatment, generallyincluding heating the bucket in vacuum to a temperature of from about2260° F. to about 2400° F. for 2 to 4 hours, so that the bucket has agamma prime fraction of greater than about 50%, or even greater than60%. The bucket may then be cooled by a furnace cool in vacuum, argon orhelium at a cooling rate of about 15° F. to about 45° F./minute to about2050° F., followed by gas fan cooling in vacuum, argon or helium atabout 100° F./minute to about 150° F./minute to about 1200° F. or below.Once below about 1200° F., the bucket(s) can be cooled to roomtemperature at any cooling rate. The bucket(s) may then undergo aging byheating under vacuum to about 1975° F. for a period of 4 hours, furnacecooling to below about 1200° F., heating to from about 1600° F. to about1650° F. for 4 to 16 hours, followed by a furnace cool to roomtemperature.

Although the superalloy of the present invention is ideally suited fordirectionally solidification casting, it can be readily produced byconventional casting or single crystal casting techniques. Thesuperalloy is well suited for high temperature turbine components suchas blades, buckets, vanes, and the like for gas turbine engines.

The following examples, which are meant to be exemplary andnon-limiting, illustrate compositions and methods of manufacturing someof the various embodiments of the nickel-based alloys. In the followingexamples, test specimens were cast in a directional solidificationfurnace. The mold withdrawal rate, which corresponds to thesolidification rate, was 12 inches per hour. Material properties weremeasured in the as-directionally solidified condition with the expressintent of optimizing chemistry independent of heat treatment effects.

Example 1

In this example, forty unique nickel-base superalloys were directionallycast and evaluated. Key material attributes required for optimal gasturbine bucket performance were identified, prior to mechanical testing.Each attribute was assigned a weighting factor based on its relativeimportance. Calculated and measured properties were then merged to acommon unitless scale, and weighted accordingly. The summation of theweighted, unitless attributes provided a means to rank alloys based ontheir total balance of properties. Table 1 indicates the chemistry ofthree exemplary alloys (Alloy 1, Alloy 2, and Alloy 3) by weightpercent, with the balance being Ni and impurities. Each of thesenickel-base superalloys had a predicted gamma prime mol fraction greaterthan 50%. Also included is a standard high temperature nickel basesuperalloy, Rene' N4, currently employed for the manufacture of hightemperature turbine components.

TABLE I Alloy Cr B C Co Al Hf Mo Nb Ta Ti W *Rene′ N4 9.75 0.004 0.057.50 4.20 0.15 1.50 0.50 4.80 3.50 6.00 1 9.59 0.01 0.07 3.93 4.13 0.150.74 0.49 5.90 3.44 8.85 2 9.78 0.01 0.04 4.01 4.71 0.15 1.50 0.50 6.012.62 6.02 3 10.00 0.01 0.05 4.10 5.80 0.50 2.50 0.00 4.45 0.82 7.97Compositions are in weight percent, with the balance being Ni andimpurities *Comparative Example

Table II provides various calculated properties of the superalloycompositions. Each alloy is predicted to exhibit a heat treatment windowsimilar to or greater than the reference alloy, Rene' N4, with improvedprocessability and yield a likely consequence. The calculated density ofeach alloy is similarly aligned with the reference alloy. Predictedgamma prime mol fraction is higher in each case, relative to Rene' N4,which is typically desirable from a high temperature strengthperspective.

TABLE II Calculated Calculated Calculated Calculated Gamma IncipientHeat Gamma Prime Melting Treatment Calculated Prime Solvus Point WindowDensity Max. NP Alloy (° F.) (° F.) (° F.) (lb/in³) (%) *Rene′ N4 21922351 159 0.298 69 1 2185 2352 167 0.302 72 2 2211 2355 144 0.299 73 32222 2381 159 0.298 76 *Comparative example

Table III summarizes various material properties measured in theas-directionally solidified (as-DS) condition, wherein the term “UTS”refers to ultimate tensile strength; and the term “YS” is yieldstrength.

Castability was analyzed by a casting cracking test in accordance withU.S. Pat. No. 4,169,742, wherein the total crack length was measured atthe outer diameter of a directionally solidified thin wall casting(about 60 mils thick). Alloys exhibiting the least amount of crackingare preferred. Each alloy in Table III exhibits superior resistance tocasting cracking, under the constraints of this screening experiment,relative to the reference alloy.

The creep behavior of each alloy was evaluated in air at 1400° F. and1800° F. Dead weight loading was used to impose a stress of 107 ksi at1400° F., and 31 ksi at 1800° F. Plastic strain was monitored throughoutthe duration of the test. Table III indicates improvements in the timesto 2% creep, which range from 2.0× to 3.5× at 1400° F., relative toas-DS Rene' N4. Additionally, improvements in the times to 2% creep at1800° F. are between 2.75× and 4.75×, relative to as-DS Rene' N4. Timesto rupture at each temperature are also improved by similar orders ofmagnitude, relative to as-DS Rene' N4.

The tensile behavior of each material was evaluated in air at 1400° F.Specimens were pulled to failure at a fixed displacement rate of 0.02inches/minute. Table III indicates a range of behavior, relative to thereference alloy. Alloys 1 and 2 exhibit considerable improvements inyield strength, with comparable ultimate tensile strengths. Alloy 3measures slightly lower in yield and tensile strengths, relative toas-DS Rene' N4. It is effectively balanced, however, by its superiorcastability and creep behavior.

TABLE III Measured Thin Wall Casting Measured Time Measured Time Totalto 2.0% Creep to Rupture Tensile Tensile Crack Strain (hours) (hours) YS@ UTS @ As-DS Length 1400° F./ 1800° F./ 1400° F./ 1800° F./ 1400° F.1400° F. Alloy  (inches) 107 ksi 31 ksi 107 ksi 31 ksi (ksi) (ksi)*Rene′ N4 17.4 23.0 14.6 95.7 25.4 146 168.6 1 13.6 82.0 69.4 133.2104.9 154.6 166.7 2 9.1 47.0 61.5 105.2 87.9 153.5 169.6 3 7.8 61.0 40.1142.4 64.0 131.2 153.8 *Comparative example

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method of making a cast and heat treated article having a gammaprime volume fraction comprising: providing a nickel-based alloycomprising about 7.0 wt % to about 12.0 wt % chromium, about 0.1 wt % toabout 5 wt % molybdenum, about 0.2 wt % to about 4.5 wt % titanium,about 4 wt % to about 6 wt % aluminum, about 3 wt % to about 4.9 wt %cobalt, about 6.0 wt % to about 9.0 wt % tungsten, about 4.0 wt % toabout 6.5 wt % tantalum, about 0.05 wt % to about 0.6 wt % hafnium,niobium present in an amount of up to about 1.0 wt %, boron present inan amount of up to about 0.02 wt %, and carbon present in an amount ofup to about 0.1 wt %, remainder being nickel and incidental impurities;melting and directionally solidifying the alloy to produce an article;and heat treating the article at about 2260° F. to about 2400° F. toform a heat treated article having a gamma prime volume fraction ofgreater than about 50%.
 2. The method of claim 1 wherein the carbon ispresent in an amount of about 0.04 wt % to about 0.1 wt. %.
 3. Themethod of claim 1 wherein the alloy has an aluminum to titanium ratio ofgreater than about
 1. 4. A method of making a cast and heat treatedarticle having a gamma prime volume fraction comprising: providing anickel-based alloy comprising about 9.0 wt % to about 11.0 wt %chromium, about 0.5 wt % to about 3.0 wt % molybdenum, about 0.5 wt % toabout 3.5 wt % titanium, about 4 wt % to about 6 wt % aluminum, about3.5 wt % to about 4.25 wt % cobalt, about 6.0 wt % to about 9.0 wt %tungsten, about 4.0 wt % to about 6.5 wt % tantalum, about 0.05 wt % toabout 0.5 wt % hafnium, niobium in an amount of up to about 1.0 wt %,boron in an amount of up to about 0.01 wt %, and carbon present in anamount of up to about 0.07 wt %, balance being nickel and incidentalimpurities; melting and directionally solidifying the alloy to producean article; and heat treating the article in a vacuum to form a heattreated article having a gamma prime volume fraction of greater thanabout 50%.
 5. The method of claim 1 wherein the gamma prime fraction isgreater than 60%.
 6. The method of claim 4 wherein the gamma primefraction is greater than 60%.
 7. The method of claim 1 wherein the alloyis substantially free of rhenium.
 8. The method of claim 4 wherein thealloy is substantially free of rhenium.
 9. The method of claim 1 whereinthe alloy comprises about 9.59 wt % to about 9.78 wt % chromium, about0.74 wt % to about 1.5 wt % molybdenum, about 0.82 wt. % to about 4.5 wt% Ti, about 4.13 wt % to about 4.71 wt % aluminum, about 3.93 wt % toabout 4.01 wt % cobalt, about 6.02 wt % to about 8.85 wt % tungsten,about 5.90 wt % to about 6.01 wt % tantalum, about 0.15 wt % hafnium,about 0.50 wt % niobium, about 0.01 wt % boron, and about 0.04 wt. %carbon, remainder being nickel and incidental impurities.
 10. The methodof claim 9 wherein the alloy comprises titanium present in an amount ofabout 2.62 wt. % to about 3.44 wt. %.
 11. The method of claim 1 whereinthe heat treating is performed by heating in a vacuum for 2 to 4 hours.12. The method of claim 4 wherein the heat treating is performed byheating in a vacuum for 2 to 4 hours.
 13. The method of claim 1 whereinthe article is a component of a gas turbine engine.